21.

A charge Q is uniformly distributed over a long rod AB of length L as shown in the figure. The electric potential at the point O lying at a distance L from the end A is

• 
• 
• 
• 

The diameter of the plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 10⁸ m/s, the focal length of the lens is 

• 15 cm

• 20 cm

• 30 cm

• 30 cm

A circular loop of radius 0.3 cm lies parallel to a much bigger circular loop of radius 20 cm. The centre of the small loop is on the axis of the bigger loop. The distance between their centres is 15 cm. If a current of 2.0 A flows through the smaller loop, then the flux linked with bigger loop is 

• 9.1 x 10^-11 Wb

• 6 x 10^-11 Wb

• 3.3 x 10^-11 Wb

• 3.3 x 10^-11 Wb

A diode detector is used to detect an amplitude modulated wave of 60% modulation by using a condenser of capacity 250 pico farad in parallel with a load resistance 100 kilo ohm. Find the maximum modulated frequency which could be detected by it. 

• 10.62 MHz

• 10.62 kHz

• 5.31 mHz

• 5.31 mHz

A beam of unpolarized light of intensity I₀ is passed through a Polaroid A and then through another polaroid B which is oriented so that its principal plane makes an angle of 45° relative to that of A.The intensity of the emergent light is

• I₀

• I₀/2

• I₀/4

• I₀/4

Two coherent point sources S₁ and S₂ are separated by a small distance ‘d’ as shown. The fringes obtained on the screen will be:

• point

• straight lines

• Semi-circle

• Semi-circle

The anode voltage of a photocell is kept fixed. The wavelength λ of the light falling on the cathode is gradually changed. The plate current I of the photocell varies as follows:

The I-V characteristic of an LED is

In a hydrogen like atom, electron makes the transition from an energy level with quantum number n to another with a quantum number (n – 1). If n >> 1, the frequency of radiation emitted is proportional to 

• 1/n

• 1/n²

• 1/n^3/2

• 1/n^3/2

The graph between angle of deviation (δ) and angle of incidence (i) for a triangular prism is represented by